1. Field of the Invention
The present invention relates to an image processing device and an image processing method. In particular, the present invention relates to an image processing device and an image processing method for preparing an image data for reproducing a halftone.
2. Description of Related Art
According to an earlier development, as a method for carrying out a halftone process for an image data having a brightness value, a density value or the like for each pixel unit, a dither method using a comparing operation for comparing the image data with a threshold matrix, has been used. As a so-called multilevel dither method in which a halftone process is carried out by outputting three or more multiple levels as output values, the method in which the above-described comparing operation are carried out a plurality of times, has been used.
According to the above-described method, in order to transform an input image data ton multiple levels, the comparing operation should be carried out n−1 times. For example, in order to transform the data to one of 64 multiple levels, it is required to carry out the comparing operation 63 times. Further, sixty-three threshold matrixes and sixty-three output values are required. Therefore, a memory area for storing these values is required. In case that a hardware for such a method is provided, the scale of the circuit becomes larger as the number of the output values increases. There was a problem that the cost thereof was increased.
On the other hand, the multilevel dither method in which an input image data is transformed to one of the multiple levels without using the comparing operation for comparing the input image data with the threshold value, is disclosed in, for example, Japanese Patent Application Publications No. Tokukai-Sho 63-125064 and No. Tokukai-Hei 6-90350.
In this method, the result of the calculation, such as the comparing operation for comparing the image data with the threshold matrix and the like, is previously obtained to store the result of the calculation as a lookup table in the circuit. The input image data is inputted to the lookup table pixel by pixel to extract the corresponding result of the calculation. By outputting the result, the image data is transformed to one of the multiple levels. According to this method, because all of the results of the calculation are stored in a memory and a comparing operation circuit is not required, there is an advantageous merit that the multiple levels or the dot patterns of the output image can be freely changed by only rewriting the results of the calculation without changing the circuit arrangement.
However, in this circuit arrangement, because all of the calculation results to be obtained are stored, large quantities of memory should be used. Therefore, even in this circuit arrangement, there was a problem that the cost thereof was increased.
Further, in an electrophotographic process type of printer or the like, because a laser, a light emitting diode (LED) or the like is actuated to emit, pulses having various widths are generated in accordance with an image signal when pulse widths of the signal which is processed by the multilevel dither method are modulated by the pulse width modulation (PWM) process. In the pulses, when the width of the output pulse is narrower than a predetermined width, the phenomenon that an LED array or a laser cannot respond to the signal nor emit, is caused. In a developing process for an electrophotography, on a photosensitive member exposed by an LED array or a laser actuated in accordance with a narrow pulse, a phenomenon that a toner cannot be stably developed is caused. By these phenomena, the problems that the image quality, in particular, the gradation property was deteriorated, was caused.